Plenty of novels throughout history have been deemed un-filmable â€“ and only a handful have actually borne out the label â€“ but there aren't many that have produced the specific challenges now facing Fifty Shades of Grey writer Kelly Marcel, director Sam Taylor-Johnson and producers Dana Brunetti and Michael De Luca. For all the sociological analysis and broadsheet think pieces that EL James's fanfiction-turned-international-bestseller has spawned, its success remains at least in part a mystery.

So what exactly is it going to take for that success to be replicated on the big screen? Needless to say, there's a degree of financial certainty for Focus Features from the get-go - people are absolutely, without question, going to go and watch Charlie Hunnam and Dakota Johnson getting kinky in Fifty Shades of Grey - but it's unclear exactly how many of the 70 million copies sold worldwide will translate into purchased tickets.

The promised NC-17 certification is the most obvious stumbling block, immediately cutting out a substantial portion of the audience that traditionally make or break a film's box office performance. It remains to be seen whether Marcel will be willing or able to stick to her guns, if the studio are determined to push for a lower rating, but going the whole hog with the explicit scenes is more or less the only way this thing can work.

Because let's be clear: Fifty Shades of Grey is not a layered story populated with three-dimensional characters and compelling dramatic incidents. It's a series of increasingly extreme sex scenes punctuated by half-hearted gestures towards plot and backstory. Any conversation that occurs between characters exists only as a stepping-stone to the next bit of boning. The sex scenes - shoddily written and littered with weird metaphors though they are - are the only aspect of the book that works. Bluntly, if the sex isn't hot, there's nothing to fall back on here.

But if the makers of Fifty Shades are able to preserve the erotic stuff, there's potential for the film to be substantially better than the book. What's really galling about the book is James's tortuous and clunky prose - just take a look through these highlights if you don't believe us - and unless Marcel has gone for a voiceover, all that will be excised on-screen.

It's becoming increasingly common for young-adult novels - including The Hunger Games - to be written in the first-person present tense, which creates a sense of immediacy. Fifty Shades, while arguably not aimed at young adults, is another example of this, and what it means for Hollywood adaptations is that a lot of the main character's depth will inevitably be lost. In the case of The Hunger Games this was a bad thing, because Katniss Everdeen is a nuanced and interesting character whose inner life has resonance. In the case of Fifty Shades of Grey, it could be a blessing, because Anastasia Steele is a cardboard cutout whose inner monologue is excruciating to read.

It's not just limited to the occasional aside, although lines like "Holy crap!", "Damn my hair!" and "Get a grip!" are used liberally. The weirdest aspect of James's characterization of virginal college student Ana is the Inner Goddess/Subconscious device.

The Inner Goddess is her libido, personified as a hyperactive and disturbingly childlike nymph-creature who's always doing samba moves whenever something racy happens. It works exactly as well on the page as you're imagining. Meanwhile her Subconscious is a sneery stick-in-the-mud who thoroughly disapproves of Ana's newfound sex drive. Losing this bipolar pair will be an easy and beneficial move for Marcel.

There are still numerous problems, mind you. Plenty of Fifty Shades' non-erotic scenes that are silly on the page - take Ana getting turned on by watching her soon-to-be dominator Christian Grey pick out lengths of rope from a supply store, or literally falling into his arms during their first meeting - will be flat-out hilarious on screen without some serious tweaking.

It's hard to imagine how The Contract, a lengthy document outlining the limitations of the dom-sub relationship Christian proposes to Ana, will be translated; we're picturing a shot of Johnson's face becoming gradually more horror-struck in a scene that culminates with the cry "He wants to whip me with what?" The risk of unintentional comedy will be high.

Fifty Shades of Grey has a uphill battle to work on screen, because it's building on foundations that are shaky at best. But given the involvement of an acclaimed director in Taylor-Johnson and a pair of serious and established producers in Brunetti and De Luca, there is reason to believe Fifty Shades, the movie, is being produced as more than a cynical money-spinning exercise. Despite the predictable fan tantrum that was thrown over this week's casting announcements, this is one adaptation with an above-average chance of ending up better than its source material.